Production of maleic anhydride

ABSTRACT

Maleic anhydride is produced by passing a C 4  to C 12  hydrocarbon stream together with molecular oxygen over a fixed bed containing a vanadium molybdate catalyst promoted with a novel cobalt tantalate having the formula 
     
         CoTa.sub.b O.sub.x 
    
     wherein b is a number from about 1.0 to about 4.0 and x is a number from about 3.5 to about 11.0. The catalyst is prepared by calcining a slurry containing cobalt oxide and tantalum oxide, or their precursors, in at a temperature range from about 525° to about 1200° C. for about 1 to about 40 hours to obtain a cobalt tantalate, which is then slurried with a mixture of oxides of vanadium and molybdenum, or their precursors, dried, and then calcined at a temperature from about 300° to about 650° C. for about 0.5 to about 24 hours. A support can be used for the catalyst.

CROSS REFERENCE TO RELATED APPLICATION

Reference is made to U.S. patent application Ser. No. 245,255 to R. A.Innes and A. J. Perrotta entitled "Cobalt Niobate, Catalyst PromotedTherewith, Preparation Thereof, Production of Maleic Anhydride UsingSaid Catalyst" filed on even date, which application is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a novel cobalt tantalate, a cobalttantalate-promoted catalyst, a method for preparing such catalyst and aprocess for producing improved yields of maleic anhydride using suchcatalyst. More particularly, the invention relates to a novel cobalttantalate, a vanadium molybdate catalyst promoted with cobalt tantalateand to the production of maleic anhydride by oxidation of a C₄ -C₁₂hydrocarbon stream using a cobalt tantalate-promoted vanadium molybdatecatalyst.

DESCRIPTION OF THE PRIOR ART

The oxidation of hydrocarbons to produce maleic anhydride using acatalyst system comprising a vanadium oxide, a molybdenum oxide ormixtures thereof is well known. More recently, such catalyst systemshave included materials in addition to the vanadium and/or molybdenumoxides, e.g., phosphorous, silica or titanium, and are discussed in U.S.Pat. No. 4,113,745 entitled "Catalyst for and Method of Producing MaleicAnhydride" to Strojny et al, which describes a process for obtainingmaleic anhydride wherein a C₅ hydrocarbon stream is oxidized with anoxygen-containing gas over a catalyst mixture containing 30 to 60 weightpercent vanadium oxide, 5 to 40 weight percent molybdenum oxide and 25to 60 weight percent titanium oxide on an alpha alumina or aluminasilica support having a surface area of less than one square meter pergram. The presence of titanium oxide in selected amounts in the catalystapparently results in higher maleic anhydride yields and permitsslightly lower temperatures during the reaction.

SUMMARY OF THE INVENTION

A cobalt tantalate-promoted vanadium molybdate catalyst has now beenfound which provides improved yields of maleic anhydride from the vaporphase reaction of a hydrocarbon stream with molecular oxygen. Thus, thecobalt tantalate-promoted vanadium molybdate catalyst of the presentinvention can provide higher yields of maleic anhydride than areobtained using a titanium dioxide-promoted vanadium molybdate catalyst.

The catalyst of the present invention comprises a mixture of vanadiumoxides and molybdenum oxides, herein referred to as "vanadiummolybdate," promoted with a novel cobalt tantalate composition havingthe formula:

    CoTa.sub.b O.sub.x

wherein b is an integer in the range from about 1.0 to about 4.0, and xis an integer in the range from about 3.5 to about 11.0. The cobalttantalate can be a single crystalline phase having the formula CoTa₂ O₆or can be a combination of crystalline phases. On an elemental basis,the molar ratio of vanadium to molybdenum can be in the range of about2:1 to about 10:1. The weight ratio of the vanadium molybdate to thecobalt tantalate can be in the range of about 1:3 to about 3:1.

The cobalt tantalate-promoted catalyst of the present invention isprepared by calcining an admixture of a cobalt compound with a tantalumcompound in the presence of an oxygen-containing gas to produce thedesired cobalt tantalate. Calcination of the cobalt tantalate must beconducted at a temperature in the range of between 525° to about 1200°C. to obtain a highly active catalyst for the production of maleicanhydride. The resulting cobalt tantalate is mixed with a vanadiumcompound and a molybdenum compound, and the mixture is then calcined inthe presence of an oxygen-containing gas at a temperature in the rangeof between about 300° to about 650° C.

The process of the present invention comprises passing a vaporousmixture comprising at least one C₄ to C₁₂ hydrocarbon and molecularoxygen at elevated temperature and pressure over the cobalttantalate-promoted catalyst to produce high yields of maleic anhydride.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As previously indicated, the catalyst of the present invention ispromoted with a novel cobalt tantalate of the formula:

    CoTa.sub.b O.sub.x

wherein b is a number ranging from about 1.0 to about 4.0, preferablyfrom about 1.5 to about 3.0, and x is a number ranging from about 3.5 toabout 11.0, preferably from about 4.5 to about 8.5. The molar ratio ofvanadium to molybdenum can be in the range of about 2:1 to about 10:1,preferably about 3:1 to about 7:1 on an elemental basis. The weightratio of the vanadium molybdate to the cobalt tantalate can be in therange of about 1:3 to about 3:1, preferably about 1:2 to about 2:1.

The novel cobalt tantalate promoter is prepared by calcining a mixtureof a cobalt compound and a tantalum compound in the presence of amolecular oxygen-containing gas, such as air, substantially pure oxygenor the like, at a temperature in the range of about 525° to about 1200°C., preferably from about 750° to about 1000° C., for about one to about40 hours, preferably from about 8 to about 24 hours. Any suitablepressure can be employed, since calcination pressure is not critical.Thus, the pressure can be as high as desired, for example, up to about50 pounds per square inch gauge (345 kPa) or even higher. However,atmospheric pressure is preferred. The aforesaid calcinationtemperatures must be utilized to produce a highly active catalyst. As aresult of such calcination, the cobalt tantalate forms crystallinephases that render the final catalyst extremely effective for obtaininghigh maleic anhydride yields.

Suitable cobalt and tantalum compounds include, for example, cobaltoxide, tantalum oxide and compounds which form such oxides undercalcination conditions, such as cobalt nitrate [Co(NO₃)₂.6H₂ O], cobaltoxalate [CoC₂ O₄ ], cobalt hydroxide [Co(OH)₂ ], cobalt acetate [Co(C₂H₃ O₂)₃ ], tantalum hydrogen oxalate [Ta(HC₂ O₄)₅ ], tantalic acid [Ta₂O₅.XH₂ O], etc. The admixture of the cobalt compound and tantalumcompound can be calcined in the form of an aqueous solution or slurry.

The cobalt tantalate promoter is then admixed with vanadium andmolybdenum compounds so as to form an aqueous slurry. Suitable vanadiumand molybdenum compounds include oxides of vanadium and molybdenum, orcompounds which form such oxides including ammonium paramolybdate[(NH₄)₆ Mo₇ O₂₄.4H₂ O], molybdic acid [MoO₃.X H₂ O], ammonium vanadate[NH₄ VO₃ ], vanadyl oxalate [VOC₂ O₄ ], etc.

The slurry is then evaporated to dryness, oven dried (80° to 200° C.)for about 4 to about 24 hours, and then calcined in a molecularoxygen-containing gas, e.g., air, at an elevated temperature, forexample, in a range of about 300° to about 650° C., preferably about400° to about 500° C., for about 0.5 to about 24 hours, preferably about1 to about 5 hours, at a pressure similar to the initial calcinationpressure. Preferably, the catalyst of the present invention is supportedso as to provide for an attrition-resistant catalyst. Refractory oxides,such as alumina, silica, zirconia or mixtures thereof, such asalumina-silica, etc., are especially preferred. The supports mustpossess a low surface area, for example, in the range of about 0.1 toabout 10 square meters per gram, preferably from about 0.2 to about 1square meter per gram. When supports are used, the weight ratio ofcobalt tantalate-promoted vanadium molybdate to support can be in therange of about 1:3 to about 1:50, preferably about 1:5 to about 1:15. Ifa support is used, the support is coated or impregnated with the slurrycontaining the cobalt tantalate and the vanadium and molybdenum entitiesbefore evaporation, drying and calcination. The amounts of materialsused in preparing the catalyst are those amounts sufficientstoichiometrically to obtain a catalyst containing the elements in thedefined amounts.

The preparation of maleic anhydride according to the process of thepresent invention involves passing a C₄ to C₁₂ hydrocarbon stream,preferably a C₄ to C₁₀ hydrocarbon stream, preferably containingsubstantial amounts of unsaturated hydrocarbons containing at least twodouble bonds, for example, 1,3-butadiene, cyclopentadiene,dicyclopentadiene, piperylene, isoprene, etc., together with molecularoxygen in vapor phase at elevated temperatures and elevated pressuresover a fixed bed containing the novel catalyst herein.

The relative amounts of hydrocarbon and molecular oxygen can be variedover a wide range. However, in general, the mixture of hydrocarbon andmolecular oxygen will contain from about 0.2 to about 2 mol percent ofhydrocarbon, preferably from about 0.5 to about 1.5 mol percent ofhydrocarbon. The mixture is passed over the catalyst at a weight hourlyspace velocity, that is hydrocarbon to catalyst, ranging from about 0.05to about 1.0, preferably from about 0.1 to about 0.5.

Temperatures which are suitable for use in the present process are thosetemperatures which initiate a reaction to produce maleic anhydride andare generally from about 300° to about 500° C., preferably about 375° toabout 450° C. Pressures are not critical and can be as high as about 50pounds per square inch (345 kPa), or even higher, but in general aboutatmospheric pressure will suffice.

The recovery of maleic anhydride from the reaction product can beeffected in any suitable manner. For example, the product can becondensed and then scrubbed with water, and the maleic anhydriderecovered from the aqueous solution by evaporation and/orcrystallization.

The following examples illustrate the catalyst, preparation of suchcatalyst and the process for producing high yields of maleic anhydrideusing such catalyst.

EXAMPLE I

A cobalt tantalate promoter was prepared by first dissolving 5.82 gramsof cobalt nitrate [Co(NO₃)₂.6H₂ O] in 100 milliliters of water. Thesolution so obtained was then added to 8.83 grams of tantalum oxide [Ta₂O₅ ] and the resulting mixture was stirred for 15 minutes. The mixturewas then transferred to an evaporating dish and heated to 90° C. todryness, and the resulting material was calcined in air at atmosphericpressure at 900° C. for 16 hours to yield a crystalline material havingthe formula CoTa₂ O₆ and an x-ray diffraction pattern with the principalpeaks set forth below in Table I.

                  TABLE I                                                         ______________________________________                                        d(A°)      I/I.sub.o                                                   ______________________________________                                        4.57              10                                                          4.19              15                                                          3.33              100                                                         2.56              73                                                          2.36              24                                                          1.74              54                                                          1.67              15                                                          ______________________________________                                    

In the above Table I, d(A°)=diameter in Angstroms, I=intensity and I_(o)=maximum intensity.

The aforesaid procedure was repeated, except that a lower calcinationtemperature of 550° C. was used to produce a second sample of cobalttantalate promoter in which cobalt and tantalum oxides were onlypartially reacted to form a mixture of crystalline phases.

EXAMPLES II-VIII

For purposes of comparison, four vanadium molybdate catalysts wereprepared. The first catalyst was prepared by mixing 2.04 grams ofammonium vanadate [NH₄ VO₃ ] and 0.66 gram of ammonium paramolybdate[(NH₄)₆ Mo₇ O₂₄.4H₂ O] in 100 milliliters of water. The mixture wasstirred for 15 minutes and then transferred to an evaporating dish towhich 26.35 grams of silica-alumina spheres having a surface area ofabout 0.75 square meter per gram were added and then mixed at 90° C.until dryness. The resultant slurry was oven dried overnight at 120° C.and then calcined in air at atmospheric pressure and a temperature of500° C. for 2 hours. After calcining the spheres were crushed and sievedto obtain 10 to 20 mesh particles.

A titanium dioxide-promoted catalyst was prepared by repeating theaforesaid procedure, except that 2.13 grams of titanium oxide (TiO₂)were mixed with the ammonium vanadate, ammonium paramolybdate and water.

Two cobalt tantalate-promoted catalysts were then prepared by repeatingthe aforesaid procedure except that 2.13 grams of the cobalt tantalate[CoTa₂ O₆ ] of Example I that had been calcined at 900° C. and 2.13grams of the cobalt tantalate sample that had been calcined at 550° C.,respectively, were each mixed with the ammonium vanadate, ammoniumparamolybdate and water in two separate experiments to obtain two cobalttantalate promoted catalysts, each containing approximately 7.0 weightpercent vanadium molybdate (V₄.7 MoO₁₄.75), 7.0 weight percent cobalttantalate (CoTa₂ O₆) and 86 weight percent silica alumina support.

Multiple tests were conducted wherein 10 grams of each catalyst asprepared above was diluted to a volume of 45 milliliters with 10 to 20mesh quartz and charged to a 3/4 inch tubular stainless steel reactor.The reactor was immersed in a fluidized alundum bath and heated toreaction temperature with air flowing over the catalyst bed. A singlehydrocarbon or a mixture of hydrocarbons were then pumped into theheated line to produce a feed stream containing one mol percenthydrocarbon at a weight hourly space velocity of 0.2. After a suitableline-out period, the product was collected in a cold finger trap at -78°C. and then passed to a scrubber containing water. At the end of the runthe scrubber solution was added to the cold finger trap to dissolve themaleic anhydride. The total acidity of the final solution was determinedby potentiometric titration with 0.1 N NaOH. The percentage of the totalacidity attributable to maleic anhydride was then determined by gaschromatagraphic analysis of the scrubber solution, and the weightpercent yield was calculated.

Four hydrocarbon streams were used in the runs. Stream A contained morethan 95% by weight dicyclopentadiene. Stream B contained only butadiene.Stream C had the following analysis:

    ______________________________________                                        Component        Weight Percent                                               ______________________________________                                        2-methylbutene-2 20.4                                                         Piperylene       47.7                                                         Dipentenes Plus                                                               Dicyclopentadiene                                                                              21.8                                                         Other C5's*      10.1                                                         ______________________________________                                         *Isoprene, Cyclopentadiene, Cyclopentene, Pentyne1, Cis and Transpentene-                                                                              

Stream D had the following analysis:

    ______________________________________                                        Component        Weight Percent                                               ______________________________________                                        Piperylene       34.8                                                         Dicyclopentadiene                                                                              30.2                                                         Cyclopentene Plus                                                             Unanalyzed C.sub.6 's                                                                          25.4                                                         Other Unanalyzed                                                              Dipentenes       8.1                                                          Other C5's*      1.5                                                          ______________________________________                                         *Isoprene, Cyclopentadiene, Cyclopentene, Pentyne1, Cis and Transpentene-                                                                              

The results obtained are set forth below in Table II.

                  TABLE II                                                        ______________________________________                                                               Promoter        Yield of                               Ex-   Hydro-           Calcination                                                                           Bath    Maleic                                 ample carbon           Tempera-                                                                              Tempera-                                                                              Anhydride                              Nos.  Stream  Promoter ture (°C.)                                                                     ture (°C.)                                                                     (Wt. %)                                ______________________________________                                        II    A       None     --      400     58                                     III   A       TiO.sub.2                                                                              --      400     75                                     IV    A       CoTa.sub.2 O.sub.6                                                                     550     425     63                                     V     A       CoTa.sub.2 O.sub.6                                                                     900     400     86                                     VI    B       CoTa.sub.2 O.sub.6                                                                     900     400     81                                     VII   C       CoTa.sub.2 O.sub.6                                                                     900     400     76                                     VIII  D       CoTa.sub.2 O.sub.6                                                                     900     400     74                                     ______________________________________                                    

The results set forth in Table II show that in Example II, wherein thecatalyst contained solely vanadium and molybdenum oxides, the yield ofmaleic anhydride was only 58 weight percent. The addition of TiO₂ to thecatalyst, as in Example III, and in U.S. Pat. No. 4,113,745 to Strojnyet al, referred to above, was sufficient to increase substantially themaleic anhydride yield to 75 weight percent. However, substitution ofcobalt tantalate for titanium oxide in Example IV resulted in maleicanhydride yield (63 weight percent) less than that of Example III. InExample V, the cobalt tantalate promoter was calcined at 900° C., andthe yield of maleic anhydride was unexpectedly increased to 86 weightpercent.

In Examples VI, VII and VIII, hydrocarbon streams of differingcompositions were used with the cobalt tantalate promoter calcined at900° C., and the yield of maleic anhydride was substantially increasedover that in which no promoter was used with a hydrocarbon stream whichwas more favorable towards producing maleic anhydride.

We claim:
 1. A process for preparing maleic anhydride which comprisespassing a vapor mixture containing at least one C₄ to C₁₂ hydrocarbonand molecular oxygen over a catalyst comprising a cobalt tantalatepromoted vanadium molybdate under reaction conditions to obtain amixture containing maleic anhydride.
 2. The process of claim 1, whereina reaction temperature between about 300° to about 500° C. is used. 3.The process of claim 2, wherein the reaction temperature is betweenabout 375° to about 450° C.
 4. The process of claim 1, wherein thereaction pressure is between about 0 to about 50 pounds per square inch(345 kPa).
 5. The process of claim 1, wherein the weight hourly spacevelocity, based on hydrocarbon to catalyst, is from about 0.05 to about1.0.
 6. The process of claim 5, wherein the weight hourly spacevelocity, based on hydrocarbon to catalyst, is from about 0.1 to about0.5.
 7. The process of claim 1, wherein the hydrocarbon stream containsat least one C₄ to C₁₀ hydrocarbon.
 8. The process of claim 7, whereinthe hydrocarbon stream contains substantial amounts of unsaturatedhydrocarbons containing at least two double bonds.
 9. The process ofclaim 1, wherein the mixture of hydrocarbon and molecular oxygen willcontain from about 0.2 to about 2.0 mol percent of hydrocarbon.
 10. Theprocess of claim 9, wherein the mixture of hydrocarbon and molecularoxygen will contain from about 0.5 to 1.5 mol percent of hydrocarbon.11. The process of claim 1, wherein the cobalt tantalate promoter hasthe formula:

    CoTa.sub.b O.sub.x

wherein b is a number from about 1.0 to about 4.0 and x is a number fromabout 3.5 to about 11.0.
 12. The process of claim 11, wherein b is anumber from about 1.5 to about 3.0 and x is a number from about 4.5 toabout 8.5.
 13. The process of claim 12, wherein the cobalt tantalate hasthe formula:

    CoTa.sub.2 O.sub.6


14. the process of claim 11, wherein the cobalt tantalate promoter iscalcined at 750°-1000° C.
 15. The process of claim 1, wherein the weightratio of the vanadium molybdate to the cobalt tantalate is from about1:3 to about 3:1.
 16. The process of claim 15, wherein the weight ratioof the vanadium molybdate to the cobalt tantalate is from about 1:2 toabout 2:1.
 17. The process of claim 1, wherein the molar ratio ofvanadium to molybdenum is from about 2:1 to about 10:1.
 18. The processof claim 17, wherein the molar ratio of vanadium to molybdenum is fromabout 3:1 to about 7:1.
 19. The process of claim 1, wherein saidcatalyst comprises a support.
 20. The process of claim 19, wherein thesupport is alumina, silica, zirconia or mixtures thereof.
 21. Theprocess of claim 20, wherein the support is silica-alumina.
 22. Theprocess of claim 19, wherein the support has a surface area from about0.1 to about 10 square meters per gram.
 23. The process of claim 22,wherein the support has a surface area from about 0.2 to about 1 squaremeter per gram.